1. Field of the Invention
The present invention relates to a method for preparing an extrudate catalyst having a relatively homogeneous metal distribution, as compared to the product of comparable previously known methods. More specifically, the invention relates to a method for manufacturing a catalyst base material and for impregnating one or more metals on the catalyst base in a relatively homogeneous distribution.
2. Description of the Prior Art
Zeolitic catalysts containing sodium and noble metals have been offered for sale by several catalyst vendors, particularly as hydrocarbon saturation catalysts for making diesel fuels which are relatively low in aromatics hydrocarbon content. Such catalysts and methods for their manufacture are described, for example, in U.S. Pat. No. 5,147,526 issued to Kukes et al. and assigned to the assignee of the present invention and in U.S. Pat. No. 5,391,291 issued to Winquist et al. Typically, the catalysts are prepared by adding noble metals, such as platinum and palladium, through an incipient wetness impregnation technique to a Y zeolite powder which is subsequently compounded with a binder and extruded to produce an extrudate catalyst. The extrudate catalyst is dried and subjected to a heat treatment, known as calcining, which includes eating the extrudate catalyst to a temperature in excess of 500 degrees F. The sodium content of the final product is normally considered an important variable.
Catalysts prepared by adding noble metals to a zeolite powder before extruding are entirely satisfactory for many purposes. Indeed, they are widely favored for their uniform disposition of noble metals throughout the extrudate. However, the practice of adding noble metals to a zeolite powder before compounding with a binder and extruding adds to the cost of manufacture because a significant fraction of such noble metals is unavoidably lost in the course of handling the powder.
The catalyst manufacturing industry has long recognized a need for alternatives to the practice of adding noble metals to a zeolite powder before compounding and extruding. Researchers have attempted, for example, to impregnate noble metals by introducing a metal-containing impregnation solution to pre-formed extrudate blanks, which include zeolite and a binder and have previously been extruded, dried and calcined. Nevertheless, the metals distribution on extrudate catalysts prepared by impregnating pre-formed blanks is still generally unpredictable and, all too often, non-homogeneous.
U.S. Pat. No.3,779,899 issued to Mears reports an aromatic saturation catalyst prepared by impregnating platinum onto an acidic, previously stabilized Y zeolite support, and by calcining the resulting composite at temperatures above 1000 degrees F. The final sodium content reportedly should be less than 3 weight percent (as Na.sub.2 O), preferably less than 1.5 percent. The Y zeolite support is described as being stabilized by one of two general procedures, or a combination of both. In one stabilization procedure, at least about 80 percent of the original sodium ions are back-exchanged with a solution of polyvalent metal salt and then calcined. In another stabilization procedure, substantially all of the exchange sites are converted to decationized and/or hydrogen ion sites by exhaustive exchange with ammonium ions, followed by a hydrothermal treatment. Thereafter, platinum is reportedly added to the previously stabilized Y zeolite by conventional procedures. The Mears patent also reports that, in cases where the zeolite has previously been converted to a hydrogen form, it is desirable to reconvert the base to the ammonium form prior to exchange with platinum solutions. The Mears patent states that the zeolite base may be formed into the shape desired for the final catalyst either before or after the addition of platinum.
U.S. Pat. No. 3,897,327 issued to Ward describes a stabilized Y zeolite composition which is prepared from sodium Y zeolite by partial exchange of ammonium ions for sodium ions, steam calcination, and further ion exchange of sodium ions to reduce the final Na.sub.2 O content below about one weight percent. The stabilized zeolite is reportedly admixed with a finely divided refractory oxide of a difficultly reduced metal. For use as a catalyst in acid catalyzed reactions such as alkylation, isomerization, cracking, and hydrocracking, the Ward '827 patent reports that the stabilized zeolite from the second ammonium ion exchange step is subjected to a second calcination at temperatures between about 600 and 1500 degrees F. The Ward '827 patent states that a hydrogenating metal component, such as palladium or platinum, may be impregnated into the powdered zeolite after the second ammonium ion exchange, or into the catalyst pellets prior to the final calcination step. The Ward '827 patent provides no guidance for obtaining homogeneous impregnation of the metal.
U.S. Pat. No. 4,556,646 issued to Bezman describes a method of manufacture which includes impregnating a formed catalyst base with a solution of a noble metal, such as palladium and platinum, and an ammonium salt in order to prepare a noble metal and Y-type zeolite catalyst having a highly uniform radial distribution of the noble metal. The Bezman patent states that the ammonium salt is present in a concentration corresponding to at least 100%, and especially 200%, of the ion-exchange capacity of the zeolite component of the catalyst base. The impregnated base is reportedly permitted to stand between impregnating and drying for a time sufficient to produce a radially uniform distribution. However, the relatively high concentration of ammonia salt required for the method described in the Bezman patent tends to displace from the zeolite desirable nonframework cations such as, for example, sodium.
In spite of significant efforts by earlier researchers, need still exists for an improved method for preparing a uniformly impregnated extrudate catalyst including a zeolite, a noble metal, and a predetermined amount of sodium. Preferably the improved method avoids displacing desirable nonframework cations which are present on the zeolite at the time of extrusion. Most preferably, the method produces extrudates which are sufficiently hard for practical use in industrial chemical reactors.